would this work with another nut butter? Almond, for example?

JaneNaomi - if you're looking for an easy, neutral (not meat, not dairy) chocolate mousse for Passover and this one doesn't work out, consider the various recipes using avocado, coconut fat or oil, cocoa powder.
They require, if I remember rightly, no little or no heating, just mixing.
Substitute kosher for passover ingredients as needed (sweetener, vanilla).
Here are a couple examples.
https://www.foodnetwork.com/recipes/giada-de-laurentiis/chocolate-avocado-mousse-recipe-2110024
https://www.gordonramsay.com/gr/recipes/chocolate-and-avocado-mousse/

This recipe sounds great, but how do you combine the chocolate water, and not have it "seize"? Chocolate being melted, simply can't tolerate even small drops of water.
I'm afraid to try this recipe.

Hello! I haven't tried this recipe with another nut butter, so can't say for sure. Based on this recipe (https://food52.com/recipes/77576-chocolatey-tahini-cups), I imagine that a tahini substitution would work. Or, if you're looking for another nut, a sweetened almond butter would be a good place to start. But! In both cases, I'd add the water slowly and stop when it looks fluffy enough to you (you might not need as much water as with the peanut butter).